Conventionally, there are known obstacle detection devices that each detect an obstacle existing rearward of a vehicle such as an automobile by using an ultrasonic sensor (sonar) mounted on a rear portion of the vehicle. The ultrasonic sensor transmits an ultrasonic wave (transmission wave) rearward of the vehicle. The obstacle detection device receives a reflected wave obtained by the ultrasonic wave hitting the obstacle and being reflected, and detects the obstacle based on the reflected wave.
In such an obstacle detection device, when extending a distance of coverage of the transmission wave, it is considered to enlarge a transmission wave voltage gain. However, when the transmission wave voltage gain is enlarged, the transmission wave hits a road surface and is reflected, and the obstacle detection device erroneously detects an obstacle on the road surface based on the reflected wave only from the road surface.
Therefore, for example, PTL 1 discloses a technique to detect an obstacle existing on a road surface by comparing an average of a reflected wave from the road surface where the obstacle exists with an average of the reflected wave only from the road surface.